A RAID storage system has been conventionally known. The RAID storage system includes a RAID storage device and a storage controller. The RAID storage device includes at least one RAID array which has a configuration called Redundant Arrays of Independent Disks (RAID) (RAID configuration). Generally, a RAID array is formed using a plurality of hard disk drives (HDD's).
The storage controller controls access to any RAID array. Further, when a failure occurs in any one of the HDD's in one of the RAID arrays, the storage controller performs operation for restoring data held in the failed HDD based on data held in the remaining HDD's (what is called a rebuilding operation).
However, if any data cannot be normally read from any of the remaining HDD's because of an error such as a medium error, the storage controller cannot perform the rebuilding operation. That is, the storage controller cannot restore the RAID configuration in the RAID array. In such a case, the RAID array is in a multiple failure, in which at least two HDD's are in failure.
In order to avoid such a multiple failure, the storage controller patrols, using read operation, all the storage areas of all the HDD's which form a RAID array. It is the patrol that makes it possible for the storage controller to detect a medium error in advance, for example. In recent years, any HDD in any RAID array tends to increase in capacity, so that the time required to patrol any HDD (in any RAID array) also tends to substantially increase.
On the other hand, as an HDD is made to have a greater capacity, a phenomenon called weak write tends to easily occur. The term “weak write” refers to a phenomenon that some factors or other suddenly make the flying height of a head higher while the head is used for writing data on a medium (disk) of an HDD, because of which the medium will be partially insufficient in magnetization. The term “weak write” also refers to a further phenomenon that data cannot be normally read from a portion of the medium that is insufficiently magnetized at write operation as has been just mentioned. Minute particles on the medium may be enumerated as one of the factors of weak write. That is, particles may enter between a head and a medium, and may cause the flying height of the head higher, resulting in occurrence of weak write.
Medium errors caused by weak writes may be detected and restored by the above-mentioned patrol. However, as time taken by a patrol increases with enlargement in capacity of an HDD, time until a medium error is detected will also be extended. Then, it is highly possible that a RAID array may get into a multiple failure, because a medium error may occur in any one of the HDD's in the RAID array while some other one of the HDD's in the RAID array is yet subjected to a patrol to detect a medium error.